An LC tuned circuit that includes an inductor has been mainly used as a driving circuit in a piezoelectric ultrasonic wave sensor. However, electrostatic capacity change of the piezoelectric sensor based on a temperature needs to be compensated to maintain a driving waveform and driving efficiency in the LC tuned driving circuit, when the LC tuned driving circuit is used in a wide temperature range of about −40° C. to about 80° C., for example, in an ultrasonic wave sensor for a vehicle parking aid.
A temperature coefficient of capacitance (TCC) indicates a temperature compensation rate of an electrostatic capacity temperature compensation material to a reference temperature of 25° C. and is provided as follows:TCC (ppm/° C.)=106X(CT-C25/C25)/(T−25)
wherein, T indicates a temperature in Celsius (° C.), and each CT or C25 indicates electrostatic capacity at each temperature of T or about 25° C.
A piezoelectric material used for the piezoelectric ultrasonic wave sensor typically includes a lead zirconium titanate (or PZT)-5-based soft piezoelectric material having a large piezoelectric constant and a small aging of frequency. However, the PZT-5-based soft piezoelectric material may have a substantially high TCC ranging from about 2,500 and about 4,000 ppm/° C. at a temperature of about −40 to about 25° C. and about 25 to about 80° C. and also, substantially high relative permittivity of about 2,000 or more.
A piezoelectric device for the ultrasonic wave sensor may be mostly adhered to a material such as aluminum, a polymer plastic, and the like, using an adherent such as epoxy and the like. Thus, such piezoelectric device may have much greater TCC due to change in hardness which depends on a temperature of the adherent. For example, the TCC depending on temperature characteristics of an adherent may be in a range of about 6,000 to about 10,000 ppm/° C. The piezoelectric device may be coupled in parallel with a temperature compensation device in the ultrasonic wave sensor. Accordingly, an electrostatic capacity compensation device may have a range of an electrostatic capacity that is appropriately selected by considering a compensation rate to minimize a transmitting wave-type vibration decrease characteristics and maintain reception sensitivity of the ultrasonic wave sensor. Therefore, the compensation device may have electrostatic capacity in a range of about 30% to about 70% of the electrostatic capacity of the piezoelectric device.
A temperature compensation device in an ultrasonic wave sensor for a vehicle has been developed continuously. In one example, such temperature compensation device may be internally built in a sensor structure and a wire may be directly soldered thereon. Since the ultrasonic wave sensor needs a driving voltage of about 400 to about 600 V/mm increasing electrostatic capacity by decreasing a thickness may be limited when relative permittivity is small with consideration to an insulation internal pressure, a separation distance of consecutive surfaces for insulation, and the like. In addition, when the electrostatic capacity is increased by decreasing the thickness, the temperature compensation device may have substantially low strength and become difficult to handle and further to manufacture into an integrated body with the ultrasonic wave sensor.
Accordingly, to down-size, and easily handle or manufacture the temperature compensation device or obtain an effective temperature compensation of the piezoelectric ultrasonic wave sensor in a wide range of temperature, the dielectric material having a temperature compensation rate of about −5,000 to about −30,000 ppm/° C. and relative permittivity of greater than or equal to about 1000 may be required.
Currently used dielectric materials for temperature compensation for a common circuit may include a calcium titanate (CaTiO3)-zirconium titanate (ZrTiO3)-strontium titanate (SrTiO3) based material but has a temperature compensation rate of about −5,000 to about −6,000 ppm/° C. at maximum and relative permittivity of about 200 to about 800. In some examples, a barium titanate (BaTiO3)-calcium zirconate (CaZrO3)-zinc oxide (ZnO)- silicate (SiO3) based material having a temperature coefficient of capacitance (TCC) of about −5,000 to about −15,000 ppm/° C. has been developed but relative permittivity thereof may be about 700 to about 1,100. In other example, a lead oxide (Pb3O4)-strontium oxide (SrO)- calcium oxide (CaO)-titanium oxide (TiO2)-bismuth oxide (Bi2O3)-magnesium oxide (MgO) based material having a temperature coefficient of capacitance (TCC) of about −2,500 ppm/° C. and relative permittivity of less than or equal to about 500 has been developed. However, such materials may include toxic lead (Pb). In another example, a calcium titanate (CaTiO3)-lead titanate (PbTiO3)-lanthanum oxide (La2O3)-titanium oxide (TiO2) based material having a temperature coefficient of capacitance of about −8,700 ppm/° C. has been reported, However, relative permittivity thereof may be less than or equal to about 1,000 and Pb may be included as well.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.